Researchers led by Takashi Jin at Hokkaido University in Sapporo, Japan, have discovered that coating quantum dots with calixarene alters their optical properties without changing their size. “From the point of view of biological application, this technique is very useful to prepare multicolored quantum dots with similar particle sizes for fluorescent labeling probes to use in Förster resonance energy transfer [FRET] experiments,” Jin said. Scientists prefer to use similar size fluorophores in such experiments because the FRET efficiency depends on the distance between the centers of the fluorophores used. Larger fluorophores have a greater distance from their center to their outer valence. In addition, Jin said that the size of the quantum dots — less than 10 nm — may allow them to permeate the cell membrane more easily and make them less likely to interfere with normal biological processes than larger quantum dots coated with amphiphilic polymers. Using cadmium-selenide quantum dots with a zinc-sulfide shell, the researchers coated the quantum dots with calixarene carboxylic acids, with four, six or eight arenes. They added potassium tert-butoxide to deprotonate the carboxylic acids, which made the quantum dots water-soluble. Causes redshifts In the July 26 issue of The Journal of the American Chemical Society, the scientists reported that calixarene coating resulted in redshifts, depending on the position of the arene group. For example, a quantum dot with a 530-nm emission peak had redshifts of 8, 21 and 35 nm for calixarenes with four, six and eight arenes, respectively. The effect of the calixarene coat on the redshift decreased with increasing particle size. In addition, the quantum yield of the coated particles was 0.1 to 0.34, which the researchers said was high. The photostability remained unchanged for more than one month in aqueous solution. This is important because modifying quantum dots to make them water-soluble typically results in decreased photostability and quantum yield. Despite the diminished effect of calixarene coating with increasing particle size, the researchers said that their technique is useful for changing the optical properties of quantum dots without altering their size. Jin said that coating the quantum dots with calixarenes that have another chemical embedded in them can be useful for detecting metal ions and small organic compounds. The research team already has tested the utility of this method for detecting acetylcholine.